1. Technical Field
The present invention relates to a fixing member, a fixer and an image forming apparatus.
2. Description of the Related Art
A heater passing a sheet-shaped material through a nip between a fixing member and a pressing member while sandwiching the material to heat the material is known. Specific examples thereof include image heating fixers in image forming apparatuses such as electrophotographic copiers, electrostatic recorders and laser beam printers (LBP). An unfixed toner image is formed with a heat fixable toner including heat meltable resins, using any desired image forming process such as electrophotography, electrostatic recording and magnetic recording. The unfixed toner image is directly or indirectly transferred onto a recording material such as an electrofax sheet, electrostatic recording sheet, transfer material sheet or print paper. The image heating fixer fixes the unfixed toner image on the recording material with heat as a fixed image.
Conventionally, for the image heating fixers, a heat roller method is mostly used. In such a method, a recording material is transferred between a pair of rollers formed of a heat roller as a fixing member that is controlled to have a predetermined temperature and a pressure roller as a pressure member having an elastic layer and that is caused to contact the heat roller. As the recording material passes a nip formed by the rollers, a toner image is fixed on the recording material with heat and pressure.
Recently, fixing members including a low-heat-capacity heat roller having a heat source inside, a metallic belt or a heat-resistant film (fixing film) have been used to start up the fixer at high speed while saving energy. In such a heating fixer, a low-heat-capacity heating element is used to save electricity and shorten wait time.
On the other hand, when toner images are continuously fixed on small-size recording media, the fixing member has a difference in heat discharge between a heat roller part (paper passing part) a recording medium such as a print sheet contacts, and the other heat roller part (non paper passing part) the recording medium does not contact. Namely, the heat roller part that the recording media do not pass has a surface temperature higher than that of the heat roller that the recording media pass. This is mostly observed at the end portion of the heat roller and called “end temperature increase”.
Continuous excessive end temperature increase causes damages and shorter lives of the fixing member and the pressure member, and hot offset, and further unstable runnability of a recording medium such as copy papers.
Methods of increasing heat conductivity of the pressure roller to reduce the end temperature increase are typically known as disclosed in Japanese Patent No. JP-4508692-B1 (Japanese published unexamined application No. JP-2006-273771-A), and Japanese published unexamined applications Nos. JP-2009-31772-A, JP-2010-151960-A and JP-2012-37874-A.
However, heat tends to escape from the pressure roller having high heat conductivity, resulting in increase in power consumption.